The present invention relates to security systems, and more particularly, to an intruder detection system which utilizes a bistatic Doppler radar tracking scheme.
Many building complexes require heavy security, for example, nuclear processing, manufacturing and stockpiling sites. In connection with this type of installation, it is desirable to provide a detection zone which surrounds the installation and is spaced a predetermined safe distance therefrom to permit early detection of an intruder so that a timely response can be initiated. As a security installation becomes larger, the detection zone increases in size. It is then necessary to provide a capability for both detecting an intruder and tracking the intruder to facilitate interception. Such tracking also provides an indication of the intruder's intent since it often indicates an effort to enter a key building.
In order to provide adequate security, the detection zone must be substantial in size. For example, the detection zone may comprise an annular ring having an inner edge which is radilly spaced approximately eight kilometers from the center of the location which is to be protected. Furthermore, the annular ring may have a radial width of approximately one kilometer. The foregoing criteria result in a detection zone enclosing approximately one-hundred million square meters and having an outside perimeter of approximately fifty-two kilometers. Such a high security installation needs to be located in an open and unpopulated area in order to reduce the likelihood of intrusions by local residents and possibly large domestic animals.
A security system for a military, nuclear, or similarly sensitive installation should be capable of the following functions: detection of an intruder; discrimination between real threats, false alarms and nuisance alarms; generating an alarm signal and accompanying display in response to a real threat; certain control features; and deterrent capabilities. False alarms refer to those which are self-generated by internal noise of the equipment, while nuisance alarms refer to those which are generated by actual detections of external phenomena. For example, nuisance alarms can be caused by non surreptitious entry by local residents, stray cattle, various other flora and fauna, and extreme wind conditions such as high wind and lightning. As to human instrusions, there is no foolproof way to determine whether the detected individual has an aggressive intent. Therefore, it is prudent to physically intercept all detected human intruders.
Such a security system should be capable of detecting, in addition to human intruders, vehicles weighing between approximately two hundred and two thousand kilograms. Furthermore, it is desirable for such a security system to be capable of simultaneously detecting and tracking multiple intrusions.
Conventional pulse type scanning radar is inappropriate for use in security systems designed to detect intruders in a large annular surveillance zone. This is because the intruder could move toward the protected area once the beam from the rotating antenna has swept past. Utilizing an electronic sensing device, the intruder would then be able to escape detection as the beam swept past his location by, for example, hiding in a low spot in the ground.
Seismic and acoustic devices have been utilized in security systems in the past. However, often such devices require a significant amount of motion of the intruder in order to be detected. Conventional Doppler radar, such as that utilized by police, is incapable of discriminating effectively between various moving objects. Therefore, it would be insuitable for monitoring a large annular surveillance zone. Leaky transmission lines have also been utilized as intruder detection devices. They may be embedded in the soil. Interferences with reception and/or transmission which result from the lines being stepped on can be detected to determine the presence of an intruder. A further discussion of developments in security systems may be found in an article entitled "System Updates Enhance Base Security", by Kenneth J. Stein, published on page 63 of the June 29, 1981 issue of "Aviation Week & Space Technology" magazine.
In my U.S. Pat. No. 3,550,106, there is described an electromagnetic object detection system in which a sensing field is established in an area to be secured by an unobtrusive antenna. The antenna is excited by dual oscillators so as to establish a field having two frequencies. Shifts in the frequency differential of the generated signals are utilized to provide output signals which are processed to detect the intruding object.
In my U.S. Pat. Nos. 3,706,982; 3,731,305; and 3,761,908, there are disclosed similar dual frequency intrusion detection systems which are capable of discriminating against noise or other spurious effects. In my U.S. Pat. No. 3,836,960, there is described an intrusion detection system which transmits continuous wave signals in the VHF or UHF frequency bands and receives modulation components of the signals which are reradiated from metal bodies that are in contact with each other. None of the prior intrusion detection systems is suitable for continuously monitoring a large annular surveillance zone and for providing tracking information in regard to a detected intruder.